The Luna Wolves, and their later incarnation as the Sons of Horus, are one of the First Founding Space Marine Legions who get a lot of attention from pre-Heresy fans. Despite this, there is little official material from GW by way of a guide to their appearance. As the Sons of Horus, we’ve got no officially-painted miniatures and a handful of textual and artistic references.
My inspiration comes from the illustration in the Index Astartes article and a handful of references to the armour being a ‘metallic sea green’ in colour. I should mention that I’ve since seen a handful of examples painted using a Vallejo colour which is a very close (non-metallic) match for the Index Astartes illustration and looks good, although the precise name of the colour escapes me. The Collected Visions art books depict them in various tones, from pale green through green-grey to a very dark, almost black grey tone. I suppose one way to look at this is it leaves a lot of freedom for us to interpret things our own ways.